Very long optical fiber transmission paths, such as those employed in undersea or transcontinental terrestrial light wave transmission systems, employ optical repeaters along the path. As such, such fiber paths are subject to a host of impairments that accumulate along the length of the optical fiber composing the transmission path, thus causing the system performance to degrade.
Experimental evidence has shown that polarization dependent effects, induced by the optical fiber itself and/or other optical components (e.g., repeaters, amplifiers, etc.) along the transmission path, contribute to signal fading and signal-to-noise-ratio (SNR) fluctuations. One of the polarization dependent effects is termed polarization hole burning (PHB), which is related to the population inversion dynamics of optical amplifiers. PHB reduces the gain of optical amplifiers within a transoceanic transmission system for any signal having a state of polarization ("SOP") parallel to that of the primary optical signal carried by the transmission system, whereas the gain provided by these amplifiers for optical signals having an SOP orthogonal to that of the primary signal remains relatively unaffected. In simplified terms, the primary optical signal produces an anisotropic saturation of the amplifier that is dependent upon the SOP of the primary optical signal. The anisotropic saturation reduces the population inversion within the amplifier, and results in a lower gain for optical signals having the same SOP as the primary signal. This effectively causes the amplifier to preferentially enhance noise having an SOP orthogonal to that of the primary signal. This enhanced noise lowers the SNR of the transmission system and causes an increased bit error rate ("BER") .
One method of eliminating anisotropic gain saturation (i.e. polarization hole burning) in erbium-doped fiber amplifiers in optically amplified lightwave systems is to use high-speed polarization scramblers which depolarize the launched optical information signal. An example of such a prior art polarization scrambler is described in Heismann et al, "Electrooptic Polarization Scramblers for Optically-Amplified Long-Haul Transmission Systems,"IEEE Photon. Technology Letters 6, p. 1156 (1994).
Other methods to improve the transmission of optical information signals along such optically-amplified paths include the use of additional bit-synchronous phase modulation at the clock frequency of the optical information signal. This type of phase modulation improves the performance of the non-return-to-zero (NRZ) systems through partial, nonlinear conversion of the phase modulation into amplitude modulation. Typically, a single waveguide polarization scrambler generates a combination of polarization and phase modulation if the input light is linearly polarized at 45.degree.. Examples of polarization scramblers based on this type of modulation are described in U.S. Pat. No. 5,359,678, entitled "Apparatus and Method Employing Fast Polarization Modulation To Reduce Effects of Polarization Hole Burning and/or Polarization Dependent Loss," issued to Heismann et al on Oct. 25, 1994 and U.S. patent application Ser. No. 08/312,848, now U.S. Pat. No. 5,526,162, entitled "Synchronous Polarization and Phase Modulation for Improved Performance of Optical Transmission Systems," filed Sep. 27, 1994 by Bergano, both of which are incorporated herein by reference.
Generally, conventional polarization scramblers function by applying a voltage, V(t), to the drive electrode of the modulator which induces different optical phase shifts for the TE- and TM-polarized modes via the r.sub.13 and r.sub.33 electro-optic coefficients, respectively. The induced phase shift for the TM-polarized mode, .PHI..sub.TM (t), is given by: EQU .PHI..sub.TM (t)=.GAMMA.(r.sub.33 /.lambda..sub.o)V(t),
where .GAMMA. is a constant, .lambda..sub.o is the optical wavelength in free space, and r.sub.33 .apprxeq.30.8.times.10.sup.-12 m/V. Similarly, the induced phase shift for the TE-polarized mode, .PHI..sub.TE (t), is given by: .PHI..sub.TE (t)=.GAMMA.(r.sub.13 /.lambda..sub.o)V(t),
where and r.sub.13 .apprxeq.8.6.times.10.sup.-12 m/V. As represented by these equations, the two phases in these conventional phase modulators are shifted in the same direction but at substantially different rates (r.sub.33 .apprxeq.3.6r.sub.13). This gives rise to a differential TE-TM phase retardation, .PHI.(t), with .PHI.(t)=.PHI..sub.TM (t)-.PHI..sub.TE (t)=(.GAMMA./.lambda..sub.o)(r.sub.33 -r.sub.13)V(t) and to a common phase modulation, exp(j.PSI.(t)), with .PSI.(t)=(.PHI..sub.TM (t)+.PHI..sub.TE (t))/2=(.GAMMA./2.lambda..sub.o)(r.sub.33 +r.sub.13)V(t). Hence, the relative amounts of phase and polarization modulation in prior art devices are dependent upon each other by the ratio (r.sub.33 +r.sub.13)/(2(r.sub.33 --r.sub.13)).
Therefore, there exists a need in this art to provide a high-speed polarization scrambler which can adjust the phase and polarization modulation independently. In particular, there exists a need for a polarization scrambler which can produce a pure (chirp free) polarization modulation as well as a pure phase modulation. The present invention addresses these needs.